The present invention relates to a mobile telecommunication system, particularly to a method and system for reducing the average downlink transmitting power from a base station to a mobile station during a soft handover.
In a code division multiple access (CDMA) mobile telecommunication system a common frequency band is used for communicating with all base stations. Signals occupying the common frequency band are discriminated at the receiving station by a high speed pseudonoise (PN) code. Transmitter stations using different PN codes or different PN code phases produce signals that can be separately received at the receiving station.
FIG. 1 illustrates the relevant parts of a prior art CDMA mobile telecommunication system. Each base station (10, 11) transmits with a PN spreading code different from the PN spreading code of other base stations. A mobile station 20 is engaged in a conversation through the base station 10. The mobile station 20 is equipped with a receiver that allows it to measure the signal strength, in addition to the base station 10, of a group of other base stations (not shown) including the adjacent base station 11. The mobile station 20 reports the measurements to the network through the base station 10. As the mobile unit is engaged in a conversation through the base station 10 and it moves to a location, where it is able to receive the base station 11 with an adequate signal strength, a simultaneous communication path with the base station 11 is established. The decision is made by the radio network controller 30 controlling both base stations This state is called a soft handover and it is well known in the art. The mobile station is further equipped with a well known RAKE receiver, which enables the mobile station to simultaneously despread and combine the transmissions from base stations 10 and 11. In the exemplary embodiment, for the sake of simplicity, the mobile unit performs a soft handover between two base stations, but in reality more base stations may be involved.
Because CDMA base stations utilise a common frequency band for transmission, each transmitted signal is interfering with other signals. Therefore, an efficient power control for individual base stations and mobile stations is extremely important to attain the maximum capacity. Ideally each mobile should be transmitting at minimum power or at minimum energy per bit still sufficient to achieve a required signal to interference ratio (SIR) at the receiver of the receiving base station. Similarly each base station transmission directed to a specific mobile should be transmitted at minimum power or at minimum energy per bit still sufficient to achieve a required SIR at the receiver of the intended mobile unit. In the soft handover, the transmissions from both base stations directed to a specific mobile should be transmitted with a minimum power still sufficient to achieve a nominal SIR for the combined signal at the receiver of the intended mobile unit. If one base station transmits with higher power than the other, there exists virtually no diversity gain. To maximally benefit from the diversity gain, it is important to ensure that base stations involved in a soft handover transmit with powers is equal as possible. Achieving this helps to minimise the overall downlink transmission power as well.
In prior art mobile telecommunication systems mobile stations are capable of controlling base station transmission power on transmissions directed to them by generating power control commands advising the base station to either increase or decrease the transmission power by a predetermined step. The power control commands are generated responsive to measuring the SIR (or the power) of the received signal and comparing it to a predetermined threshold. The power control commands are subsequently interleaved into the uplink transmission of information. The base station receives the transmission, decodes the power control commands and adjusts it""s transmission power accordingly. This is called downlink closed loop power control. The closed loop power control may be executed in either constant or adaptive adjustment steps. One prior art method of adapting closed loop adjustment steps is described in WO 9726716.
There exists a problem in the downlink closed loop power control during a soft handover. Since the same transmission from the mobile station 20 is received at both base stations 10 and 11, both base stations will receive the same power control commands. If the received signal quality at the mobile station 20 is higher than the threshold, the mobile station 20 generates and transmits a power control command advising base stations to decrease their transmission powers. However, at the base stations, the received uplink transmissions may have experienced different attenuations and interferences causing errors at the base station 10 but not at the base station 11. Therefore, a transmission error in a xe2x80x9cdecrease powerxe2x80x9d command at the base station 10 would lead to the base station 10 increasing it""s power whereas the base station 11 would be decreasing s power as intended. This problem is called power drifting. If no correcting measures are taken, the difference in the transmission powers will last until the completion of the soft handover thus deteriorating the overall system performance.
One prior art solution to this problem is to limit the dynamic range of the downlink transmission powers. This is not a good alternative, since it results in excessive power usage by the base stations thus increasing interference.
Another known solution is to set the base station transmission powers equal at given time intervals. This is rather slow or signalling consuming solution.
Yet another known solution is to generate different power control commands for each base station. This consumes air interface resources and is not applicable in all air interfaces.
It is therefore an object of the present invention to improve the downlink signal to interference ratio by preventing the base stations involved in a soft handover from transmitting with substantially unequal transmission powers or energies per bit.
It is another object of the present invention to determine power correction parameters: a power correction interval length, a target power level or a target energy per bit level, and correction step size limits; and to signal said parameters to the network element executing a power correction algorithm.
It is another object of the present invention to correct the downlink power of the base stations involved in a soft handover with a power correction algorithm additional to downlink closed loop power control such that in the absence of closed loop power control commands the individual powers or energies per bit of separate base stations all converge to said target powers or said target energies per bit, respectively, at a rate determined by the power correction interval length and/or the correction step size limits.
It is further an object of the present invention to adjust the downlink power of individual base stations during the soft handover in such corrected steps that the possibly initially unequal transmission powers converge upon the execution of downlink closed loop power control.
It is yet another object of the present invention to provide a method to recover from the transmission errors in the downlink power control commands during a soft handover.
According to the invention, time is divided into power correction intervals. A target power or a target energy per bit and correction step limits are determined for each base station. The parameters are signalled to the base stations. Each base station subsequently executes a power correction algorithm. Each base station compares the used transmission power or energy per bit to said target power or said target energy per bit, respectively. The result of the comparison is further divided by a predetermined number of power correction steps within said power correction interval, the number preferably equal with the number of repetitions of downlink power control commands within said power correction interval, to provide a power correction step. The power correction is then executed in combination with the downlink closed loop power control. In the preferred embodiment the closed loop adjustment steps are employed at the same time with the correction steps, effectively resulting corrected closed loop adjustment steps. The corrected closed loop adjustment steps are subsequently used to adjust the transmission power according to the downlink power control commands from the mobile station within the next power correction interval. If the power control command is absent because of a sudden uplink fade, the power correction step is employed alone.